At its peak capacity, from April to September each year, the centre employs 450 staff.

IRD spent $7 million on the project, including costs associated with the temporary location in Lower Hutt, manager of transactional operations Patrick Crawford said.

The building has been earthquake strengthened to 100 per cent or more of new building standard. It had been only 34 per cent.

The interior of the building, first used by the IRD in 1989, has also fundamentally changed. The work included relocating the cafeteria, moving meeting and conference areas to the centre "and really opening up the building to catch light in the areas people are working".

The mail opening area has been made more secure, allowing a lockdown in case suspicious packages arrive. The changes followed a review of what was needed and were done with the required earthquake- strengthening work.

The IRD is paying over $1 million in rent annually for the Jepsen Gr property and has a nine-year lease.

The property, which is leased from The Wellington Company, was put up for sale last year.

It has been conditionally sold to Auckland-based Oyster Group property syndication company. The price has not been disclosed but the Wellington Company was hoping it would sell for $11 or $12 million.

Although the deal has not yet been settled, Oyster has listed the property on its website inviting interest from investors.

The company says it is seeking preliminary indications of interest in 72 shares in the property each worth $100,000.